WO2001096046A1

WO2001096046A1 - Method of producing cast calots

Info

Publication number: WO2001096046A1
Application number: PCT/GB2001/002561
Authority: WO
Inventors: Robert Batey
Original assignee: Ever Ready Limited
Priority date: 2000-06-12
Filing date: 2001-06-12
Publication date: 2001-12-20
Also published as: AU2001264074A1; GB0014407D0

Abstract

Cast calots useful for forming into zinc battery cans by impact extrusion may be produced by applying vibrations to molten zinc in a mould during cooling, wherein the application of the vibrations is initiated prior to or shortly after the onset of solidification, and is continued during solidification, of the zinc. The vibrations are applied so as to disrupt the growth of dendrites in the zinc and preferably also the surface tension forces at the zinc/air/mould boundaries, in order to produce a fine, equiaxed, noncolumnar grain microstructure associated with a substantially isotropic lead particle distribution, and preferably also a more regular cylindrical profile. The improved microstructure, and shape, of the cast calot reduces the likelihood of split defects occurring in zinc cans formed by impact extrusion, and of premature punch tool failure.

Description

METHOD OF PRODUCING CAST CALOTS

This invention relates to the production of zinc calots used for forming into zinc battery cans, more specifically cast zinc calots having improved microstructural and shape characteristics suitable for forming into cans by impact extrusion.

Zinc cans are commonly used components for certain types of cylindrical battery. In terms of production cost, the most favourable method of producing zinc battery cans is by impact extrusion from solid, cylindrically shaped pieces of cast or rolled zinc that are generally referred to as slugs or calots. In this method, preheated zinc calots are formed into the shape of open-topped, cylindrical cans by punching in an extrusion die. The as-formed extrusion product will have an excess wall portion at its open end, which portion is subsequently trimmed off to provide the cans of the specified height. To facilitate trimming by, for example, the so-called 'pinch trimming' technique, the excess wall portion may purposely be formed with a flared profile. Any defects present in the as-formed product above the trim line, such as splits in the excess wall portion, will be removed in the trimming step. However, defects present below the trim line will remain in the finished product. It is important, therefore, to ensure that no splits extend below the trim line, since these could ultimately allow oxygen into the battery during storage and thus produce a dud battery.

Cans which have been impact extruded from rolled zinc calots do not normally exhibit split defects, unless the alignment of the extrusion tool is exceptionally poor. However, cans which have been impact extruded from cast calots tend to show a higher proportion of split defects, particularly cans which have been extruded with a flared open end. Moreover, a small proportion of cans which have been impact extruded from cast zinc calots may exhibit splits that extend below the trim line. There is, naturally, concern about the level of defective cans associated with cast zinc calots relative to rolled zinc calots. Furthermore, there is concern about the relative reduction in punch tool life associated with cast zinc calots, caused by deflection of the punch during impact extrusion.

It has been recognised in the art of metal forming generally, that rolled metal deforms more uniformly than cast metal, particularly during impact extrusion, allowing greater dimensional control over the formed product and a greater freedom from defects in rolled metal extrusions. This phenomenon is largely attributable to the respective microstructures of rolled metal and cast metal. Cast zinc calots are generally produced by metering molten zinc into open graphite moulds, and allowing the zinc to cool to a cylindrically shaped solid. When the molten zinc solidifies, it crystallises to a relatively coarse microstructure consisting of large columnar grains, typically several millimetres in size. Rolled zinc is generally produced by rolling cast zinc, in a hot rolling step that recrystallises the coarse grains of cast zinc to a finer microstructure consisting of smaller, equiaxed grains, typically 0.01-0.05 mm in size. The finer, more equiaxed grain microstructure of rolled zinc flows more evenly during the deformation that takes place during impact extrusion, reducing the likelihood that the punch is deflected to cause splitting.

Another factor affecting the level of splitting in cans is the presence and distribution of lead in the calot microstructure. There is a requirement for lead in zinc for battery cans, in order to reduce corrosion of the zinc and resultant gassing during storage and discharge of the battery. Insoluble constituents in zinc, including lead, tend to segregate at the boundaries of dendrites formed in the zinc when molten zinc solidifies. The segregation or alignment of lead particles along these boundaries embrittles the zinc at elevated forming temperatures, such as those experienced during impact extrusion, causing failure by a mechanism known as 'hot shortness'. More specifically, eutectic secondary phases due to the presence of alloy metals in zinc decrease the lower limit of the hot shortness temperature range (defined as the hot forming temperature range in which the metal exhibits brittleness), so that the maximum temperature below which the metal can safely be worked without cracking is reduced. For example, for zinc which contains lead (Pb), the presence of 0.08 % Pb results in a hot shortness temperature range of 300 to 419 °C, whereas 0.3 % Pb produces a hot shortness range of 275 to 419 °C. Thus, the requirement for lead in zinc calots for battery cans limits the safe maximum forming temperature during impact extrusion and increases the likelihood of failure. The fine microstructure of rolled zinc ensures a relatively even and isotropic distribution of lead, the distribution of lead particles being intrinsically linked to the grain microstructure. However, the coarser microstructure of cast zinc enhances the segregation of lead and therefore increases the likelihood of failure still further. Thus, on the basis of microstructure, cast zinc calots are generally not as suitable for impact extrusion into battery cans as rolled zinc calots.

Moreover, rolled zinc calots are obtained by punching circular blanks from rolled metal plate stock and can therefore have a near perfect cylindrical shape, with parallel faces, a flat upper face and sharp edges, whereas cast zinc calots tend to have a less regular shape, often with nonparallel faces, a convex or concave upper face and rounded edges. It will be appreciated that a regular calot shape is desirable for impact extrusion, to ensure accurate presentation of the calot to the extrusion die and to prevent the punch from being deflected. Therefore, cast zinc calots are generally also less suitable for impact extrusion than rolled zinc calots on the basis of shape.

However, whilst rolled zinc calots may generally be more suited to impact extrusion than cast zinc calots, the production of rolled zinc calots is more costly than that of cast zinc calots, on account of the hot rolling step necessary to produce rolled metal and the high scrap loss associated with the blanking of circular calots from rolled metal plate. It would therefore be desirable to be able to produce cast zinc calots which are suitable for use in forming zinc battery cans by impact extrusion.

In a previously practised method of producing cast zinc calots, molten zinc at about 500°C was metered into open graphite moulds that had been preheated naturally to about 160°C by preceding castings. Since molten zinc tends to ball up due to surface tension, vertical vibrations were briefly applied to the mould in an attempt to level the molten zinc within the mould cavity, typically at the mains frequency (50-60 Hz) at an amplitude of 0.1-0.2 mm for 0.5-1 second. Despite this, the resulting cast zinc calots exhibited a relatively poor cylindrical profile, with a concave or convex upper surface and rounded edges. Moreover, the zinc of these cast zinc calots had a microstructure consisting of large columnar grains, typically several millimetres in size, and would therefore not flow evenly during impact extrusion.

Therefore, there remains a need for cast zinc calots that can be produced cost effectively and that can be formed into zinc battery cans by impact extrusion without exhibiting the levels of splitting and premature tool failure which have until now been associated with cast zinc calots.

What we have found, surprisingly, is that the disadvantages associated with conventional cast zinc calots can largely be overcome by disrupting the growth of dendrites in the zinc as the molten zinc cools to a solid cast. This has been achieved by applying vibrations to the zinc and controlling the vibration parameters and zinc solidification time so as to selectively transfer sufficient vibrational energy to the molten zinc during solidification that dendrite growth is disrupted.

Accordingly, in a first aspect, the present invention provides a method of producing a cast zinc calot having a fine, equiaxed, noncolumnar grain microstructure, which comprises applying vibrations to molten zinc in a mould during cooling, wherein the application of the vibrations is initiated prior to or shortly after the onset of solidification and is continued during solidification such that the growth of dendrites in the zinc is disrupted. In a preferred embodiment of this first aspect, the vibrations are applied such that they also disrupt surface tension forces at the zinc/air/mould boundaries. According to this aspect, any secondary phases such as due to the presence of lead or other insolubles in the zinc are isotropically distributed in the microstructure. As used herein, by 'isotropically distributed' is meant that the distribution of secondary phases in the zinc body is more uniform and the orientation of the secondary phases more random as compared with the distribution and orientation of secondary phases in zinc bodies of the same composition that are formed primarily of columnar grains. In a second aspect, the present invention provides a cast zinc calot as described above, having a fine, equiaxed, noncolumnar grain microstructure. Any secondary phases such as due to the presence of lead in the zinc are isotropically distributed in the microstructure. In a preferred embodiment of this second aspect, the cast zinc calot comprises an isotropic lead particle distribution that is intrinsically linked to the grain microstructure.

Additionally, in a third aspect, the present invention provides a zinc battery can formed by impact extruding a cast zinc calot as described above.

When molten zinc at the casting temperature enters the preheated mould, heat is transferred to the mould and air, and, as the zinc cools to its melting point at about 420°C, the zinc begins to solidify. In so doing, at the onset of solidification, dendrites form in the zinc and grow as the zinc continues to cool and solidifies. What we have found is that the final grain size in the solidified, cast zinc can be reduced by disrupting the growth of the dendrites as the zinc solidifies from the molten state, and that this disruption may be effected by applying vibrations to the zinc.

The point during cooling of the zinc when dendrites first appear and start to grow is herein referred to as the onset of solidification. During solidification, zinc grains are nucleated and develop, progressively from the outer zinc layer at the mould wall inwards, until at the end of solidification the zinc has fully solidified in the mould. Therefore, in order to ensure maximum disruption of the growing dendrites, and hence a small grain size throughout the body of the final solidified calot, the vibrations should be initiated before the onset of solidification and continued for as long as possible during solidification. For the smallest final grain size, the vibrations are initiated before the onset of solidification and continued until the zinc has completely solidified. Calots for AA-size battery cans have a relatively high surface area to volume ratio due to their relatively small size, and consequently cool and solidify very rapidly in the mould, by heat extraction to the mould surface and air. As a result, the onset of solidification at the calot surface tends to occur almost as soon as the molten zinc is dispensed into the mould, and thereafter the zinc solidifies rapidly inwards and into the bulk of the calot. Accordingly, when casting calots for AA-size cans we prefer that the mould is already vibrating before the zinc is dispensed into it, in order to ensure that the molten zinc is vibrated as soon as it has been dispensed into the mould, and thus before the onset of solidification, so as to disrupt dendrite growth.

For larger sized calots, for example for C- and D-size battery cans, the lower surface area to volume ratio provides a slower rate of cooling and solidification when the molten zinc is dispensed into the mould. It is therefore not usually necessary to apply vibrations to the mould already before the zinc is dispensed into the mould, as a means of ensuring that vibrations are initiated before the onset of solidification. To the contrary, for these larger sized calots, we have found that if mould vibration is active already before the zinc is dispensed into the mould, internal bubbles or surface holes may appear and be frozen in the zinc during solidification. Therefore, for larger sized calots, such as for C- and D-size battery cans, we prefer to delay applying the vibrations for a short period after the zinc has been dispensed into the mould. This delay period allows time for the escape of any air bubbles or holes trapped in the zinc on dispensing the zinc into the mould, or may simply prevent air from becoming trapped in the first place. Suitably, the delay period is from 0.1 to 2 seconds, more preferably from 0.4 to 1.1 seconds, after zinc dispense into the mould.

Moreover, for the larger sized calots, it will be appreciated that a thin surface layer of coarse, columnar grains in the solidified calot will not be detrimental to the overall flow properties of the calot during impact extrusion, provided that the bulk of the calot has the refined microstructure achieved by disruption of dendrite growth. Therefore, for larger sized calots the vibrations can be initiated shortly after the onset of solidification such that a thin surface layer, for example a surface layer of less than 1 mm thickness, has already solidified, provided that the bulk of the zinc is still unsolidified. Thus, for the larger size calots, vibrations can be delayed such that a thin surface layer solidifies first without grain refinement, provided that dendrite growth is then disrupted in the bulk of the zinc once the vibrations are initiated.

It will be appreciated that the amplitude and frequency of the vibrations can be varied provided that the overall vibrational force applied is sufficient to fracture the dendrite arms in the zinc.

Since the final grain size of the zinc is related to the number of fractured dendrite arms acting as nuclei for grain growth, it follows that the grain size may be controlled by varying the amplitude and frequency of the vibrations, and the length of time that vibrations are applied during the solidification time of the zinc. The time from the onset of solidification to the end of solidification is dependent on the rate of cooling of the zinc in the mould. Since the solidification time defines the time window during which the vibrations can be effective to disrupt the growth of the dendrites, it is important to ensure that the solidification time is sufficiently long to allow an effective amount of vibrations to be applied to the zinc.

It has been found that the rate of cooling of the zinc in the mould can be reduced, and hence the solidification time extended, by elevating the preheat temperature of the mould into which the molten zinc is dispensed. Therefore, the mould is preheated to a temperature such that the solidification time for the zinc in the mould is sufficiently long to enable an effective amount of vibrations to be applied to cause a reduced final grain size. It is preferred to elevate the preheat temperature of the mould to a temperature above 250°C, thereby retarding solidification. This temperature is substantially higher than the mould preheat temperatures currently used, which are typically in the range from 110 to 160°C. However, it has also been found that the mere application of the vibrations to the zinc will affect the solidification time, in that the solidification time is itself dependent on the amplitude and frequency of the applied vibrations. Increasing the amplitude or the frequency, or both, will tend to shorten the solidification time by, it is thought, increasing the contact area between the zinc and the mould, and thus the rate of heat dissipation, thereby accelerating the rate of cooling of the zinc in the mould. Thus, the final grain size obtained is a function of the amplitude, frequency and duration of vibration during solidification, and the preheat temperature of the mould.

In a preferred embodiment, the vibrations are applied so as not only to reduce the final grain size of the zinc by disrupting the growth of dendrites during solidification, but also to improve the shape of the zinc calot obtained when the zinc has solidified in the mould. It has been found that this may be achieved by selecting the amplitude and frequency of the vibrations such that the vibrations provide sufficient force to disrupt the surface tension forces at the boundary of the molten zinc with the mould and the air. The vibrations should be sustained for a sufficient period during solidification such that the surface tension forces are disrupted until the zinc has started to solidify at the boundary of the zinc with the mould. Thus, a zinc calot may be obtained with a sharper top edge, which may be advantageous for calot gripping and positioning in relation to the extrusion die in the subsequent impact extrusion step. Edge sharpness may be expressed in terms of the top edge radius of the calot, by which is meant the radius of curvature of the solidified meniscus at the edge of the upper surface of the calot. Accordingly, a smaller top edge radius is indicative of a sharper edge. Preferably, the vibrations are applied so as to provide atop edge radius of less than 0.7 mm, more preferably less than 0.5 mm, and particularly preferably less than 0.45 mm.

It will be appreciated that the amplitude and frequency of the vibrations may be varied provided that the vibrational force disrupts the surface tension forces to a sufficient extent to reduce the top edge radius to the desired level. However, it has been found that, above a certain amplitude, if the vibrations are sustained to the end of the solidification time, then so-called concavity defects may appear in the final calot. These manifest themselves as a concave calot upper surface, which is undesirable in that it may prevent a complete coverage of the calot with lubricant in the subsequent impact extrusion step used to form the calot into a can. Without being bound by theory, it is thought that the concave upper surface may be caused by molten zinc being thrown up near the mould wall and freezing at a higher level than zinc that is more distant from the mould wall, during the latter stages of solidification. Therefore, to avoid concavity defects, the vibrations are preferably terminated before the end of solidification, or the amplitude is so selected to provide a force that is sufficient to achieve a reduced top edge radius but insufficient to cause concavity defects.

By 'zinc' as used herein is meant pure zinc or zinc that contains one or more additives. The term 'additive' refers to any metal or compound included or present in the zinc in an amount such that it is effective for the cell, for example a metal such as lead, manganese, cadmium, bismuth, indium, calcium, aluminium or magnesium. The zinc may further comprise small amounts of impurities such as Hg, Pb, Fe, Cd, Cu, Ni, Cr, Sn, V, Al, As, Sb, Mo, Ge and ZnO, to the extent that these are not incompatible with the method of the present invention.

Preferably, the zinc is free of mercury, in the sense that it contains no added mercury, it being understood that trace impurities could be present. Preferably, lead (Pb) is present in the zinc as an additive. The total amount of the lead additive should preferably exceed 1500 ppm, more preferably exceed 2000 ppm, especially exceed 3000 ppm, and in particular exceed 4500 ppm, but preferably should not exceed 6000 ppm. We prefer that lead, if present, is present in an amount of 5000 ppm controlled to +/-300 ppm. In particular, we prefer that, in addition to lead additive, manganese (Mn) is added to a level of 150 (+/-50) ppm or cadmium (Cd) is added to a level of 300 (+/-100) ppm.

The zinc in the mould may be vibrated in any suitable manner effective to disrupt the growth of dendrites and, in the preferred embodiment, the surface tension forces at the zinc/mould/air boundaries. Preferably, the vibrations are applied mechanically by vibrating the mould using appropriate vibrating means. The mould preferably is vertically vibrated. In one practical arrangement for casting the calots, moulds are carried in succession along a conveyor, and undergo vibration by mounting and passing over a raised, continuously vibrating assembly extending along the direction of travel. On dismounting the vibrating assembly, the moulds cease vibrating and proceed further along the conveyor. In this arrangement, the duration of vibration of the zinc in the moulds may be controlled by varying the length of the vibrating assembly or speed of the conveyor. The zinc may be dispensed into each mould as or preferably just after it passes over the leading edge of the vibrating assembly. This ensures that vibration is active just prior to the zinc dispense into the mould, thus ensuring that solidification in the moulds cannot occur without the zinc being subjected to vibration. In this arrangement, since each mould will tilt forwards slightly as it dismounts from the raised vibrating assembly, the variables affecting the solidification time for the zinc should be so chosen as to ensure that the zinc does not freeze significantly when the mould is tilted. Otherwise, the molten zinc, which always tends to maintain a horizontal level, will freeze to a tapered calot having nonparallel upper and lower faces. It will be appreciated that tapered calots are undesirable for impact extrusion, as they increase the risk of the punch being deflected and causing split defects in the cans thus formed. In another arrangement, therefore, the moulds are kept horizontal, without tilting.

The molten zinc may be dispensed into the mould at any suitable casting temperature, and it will be appreciated that the casting temperature may be varied in order to control the time before the molten zinc cools to its melting point, and thus the onset of solidification. Suitable casting temperatures fall, for example, within the range of 420 to 550°C, preferably 440 to 520°C.

The present invention is applicable to any size zinc battery can formable by impact extrusion, and the appropriate amount of zinc will be selected accordingly. For an AA size zinc battery can, for example, a zinc calot weighing about 5-6 grams is generally sufficient, although somewhat higher or, preferably, lower calot weights are also envisaged for AA size zinc battery cans. The following parameter ranges apply to zinc calots for A A size battery cans, particularly zinc calots weighing less than 7.0 grams, and preferably less than 6.5 grams. The mould is generally preheated, to a temperature preferably above 160°C, more preferably within the range 250 to 350°C, and particularly preferably within the range 270 to 310°C. Suitable vibration frequencies are within the range 5 to 1000 Hz, for example 10 to 600 Hz, preferably 25 to 400 Hz, more preferably 40 to 80 Hz, and especially 50-60 Hz (mains frequency). Suitable vibration amplitudes are within the range 0.01 to 2 mm, for example 0.05 to 0.9 mm, preferably 0.2 to 0.6 mm, and especially 0.4 mm. Suitable vibration durations are within the range 0.25 to 4.0 seconds, for example 0.5 to 3.0 seconds, and preferably 2.0 seconds.

In a preferred embodiment, at 50 Hz frequency, a minimum amplitude of 0.4 mm is applied for a duration of from 0.5 to 2.5 seconds, or a minimum amplitude of 0.3 mm for a duration of from 1.0 to 2.5 seconds. In another embodiment, at 100 Hz frequency, a minimum amplitude of 0.2 mm is applied for a duration of from 1.0 to 2.0 seconds. In yet another embodiment, at 200 Hz frequency, a minimum amplitude of 0.1 mm is applied for a duration of from 1.0 to 2.0 seconds. In yet another embodiment still, at 400 Hz frequency, a minimum amplitude of 0.05 mm is applied for a duration of 1.0 second.

As mentioned above, the vibration frequency, amplitude and duration are interdependent variables which must be selected to provide a sufficient amount of sustained vibrational force during solidification of the molten zinc to disrupt the growth of zinc dendrites, and preferably also to disrupt surface tension forces at the zinc/air/mould boundaries. The amplitude and frequency of vibration, and particularly the mould preheat temperature, may be varied so as to control the solidification time in order to provide a sufficient time window for the vibrations to be applied.

From an energy consumption perspective, the method of the invention is preferably carried out using a vibration amplitude as low as possible and a mould preheat temperature as low as possible, subject to the criteria already described. Therefore, at a given frequency, there exists only a narrow range over which successful calots can be produced, given the further constraint of energy consumption. This range could not have been predicted from current knowledge.

The cast zinc calots produced in accordance with the method of the present invention have a finer, more equiaxed, noncolumnar grain microstructure, compared to conventional cast zinc calots. This renders them more suitable for forming into battery cans by impact extrusion, on account of their improved flow properties during extrusion, so that the likelihood of punch tool deflection during extrusion, and ultimately can splitting, is substantially reduced. By ensuring a finer grain structure, and consequently a larger average grain surface area, the precipitations of secondary phases along grain boundaries are more homogeneously distributed within the bulk metal body. Thus, the method of the present invention provides a further benefit in reducing the concentration of secondary phases at grain boundaries. The method further provides a second, associated benefit in reorienting the planes of the secondary phase precipitates isotropically. By providing a more randomly oriented segregation or precipitation of the secondary phases such as lead, planes of weakness which can cause brittleness are reduced. Consequently, a less brittle and more workable metal body is obtained that, again, renders the calot more suitable for forming into battery cans by impact extrusion.

Furthermore, cast calots produced in accordance with the preferred embodiment have a better cylindrical profile, with sharper edges and flatter surfaces, that facilitates accurate presentation of the calots to the extrusion die and further reduces the likelihood of punch tool deflection and can splitting.

The present invention may be further illustrated below with reference to the following examples: EXAMPLES:

Example 1

Initial trials were conducted using laboratory scale equipment, in order to determine the effect of varying the casting temperature, mould preheat temperature and amplitude, on calot shape, grain size and solidification time from zinc dispense. The results are shown in Tables 1, 2 and 3. In each case, vertical vibrations at 50 Hz frequency were applied to a mould bar carrying 6 gram zinc calot casts until the end of solidification:

Table 1

Poor A = Poor calot shape due to large top edge radius (>0.5mm). Poor B = Poor calot shape due to high concavity.

Table 1 shows that at a given frequency, if the vibration amplitude falls below a minimum amplitude, then calots having rounded top edges will result (Poor A), due to insufficient vibration force to disrupt surface tension forces. However, above a certain amplitude, concavity defects can result (Poor B) when the vibrations are applied to the end of solidification. Also, if the mould preheat temperature is too low, rapid solidification ensues and leaves insufficient time for shape refinement.

Table 2

Large = Grain size >0.2 mm OK = Grain size <0.2 mm

Table 2 shows that at a given frequency, if the vibration amplitude falls below a minimum amplitude, there is insufficient vibrational force to disrupt dendrite growth, thereby precluding grain refinement.

Table 3

Table 3 shows that the solidification time can be extended by increasing the mould preheat temperature, or by decreasing the vibration amplitude. Raising the casting temperature increases the overall solidification time (from zinc dispense), by delaying the onset of solidification.

Example 2

For this example, a 300°C mould preheat temperature was chosen as a suitable temperature at which both fine grain size and good shape could be achieved, based on the results from Example 1. The interdependence of vibration frequency, amplitude and duration, and their effects on shape and grain size, were investigated. The results are shown in Tables 4 and 5. In each case, a casting temperature of 500°C was used, and vertical vibrations were applied to a mould bar carrying 5 gram zinc calot casts.

Table 4

Poor A = poor calot shape due to large top edge radius (>0.5mm) Poor B = poor calot shape due to high concavity

Table 4 shows that, at a given frequency, a minimum amplitude is necessary to avoid a calot top edge radius of >0.5 mm (Poor A), and that increasing the frequency reduces the minimum amplitude required for a good calot shape. However, increasing the frequency may also restrict the range of possible amplitudes for good calot shape. If the duration of the vibrations is too short, the surface tension forces will be insufficiently disrupted, producing a top edge radius of >0.5 mm (Poor A). However, if the duration of the vibrations is too long, so that the vibrations are continued to the end of solidification, then concavity defects can result (Poor B). Table 5

Table 5 shows that, at a given frequency, a minimum amplitude is necessary to provide sufficient vibrational force to disrupt growing dendrites and thus refine the grain size. Increasing the frequency of vibration reduces the minimum amplitude required for successful grain refinement. Increasing the duration of the vibration tends to increase the degree of grain refinement, by increasing the number of fractured dendrite arms acting as nuclei for grain growth.

Table 6 shows the effect of frequency and amplitude on solidification time. In each case, a casting temperature of 500°C and mould preheat temperature of 300°C was used, and vertical vibrations were applied to a mould bar carrying 5 gram zinc calot casts until the end of solidification.

Table 6

Table 6 shows that the solidification time may be extended by reducing either the amplitude or the frequency of the vibrations, thereby effectively reducing the surface area of zinc in contact with the mould and slowing down the rate of cooling.

It may be seen from Example 2 that the amplitude, frequency and duration of vibration, together with the mould preheat temperature, may be selected from narrow ranges in order to obtain calots with both good grain refinement and good shape.

Example 3

The findings of the laboratory scale trials in Example 1 and 2 were applied to experiments conducted on production casting equipment, to compare a set of process values in accordance with the invention ('Best') with a set of conventionally used process values ('Current').

The 'Best' process values were selected on the following basis:

A frequency of 50-60 Hz was chosen as the vibration frequency, since this provides a relatively large range of amplitude and duration possibilities for obtaining calots of good shape and fine grain size. This frequency is also convenient being the mains frequency. A duration of 2 seconds was chosen, as a shorter time than the solidification time. An amplitude of 0.4 mm was chosen, being one of the lowest successful amplitudes for good shape and fine grain size. A mould bar preheat temperature of 280°C was used, to ensure a sufficiently long solidification time.

Graphite mould bars, each comprising two rows of eight cavities, are assembled onto steel carrier bars such that they have 1 mm of free play in the vertical plane. Approximately 200 of these assemblies are then mounted onto an endless chain conveyor. The conveyor passes along two horizontal guide rails on either side of the carrier bars. The conveyor passes beneath the zinc dispense tank and zinc is metered into each row of cavities, in turn, by the timed action of eight solenoid activated valves. A vibrating arm assembly, positioned beneath the conveyor between the guide rails and at the point of zinc dispense, vibrates two runners that are angled at about 5° to the horizontal and aligned in the direction of the travel of the mould bars. The tips of the runners are -0.5 mm above the plane of the guide rail, causing the carrier bar/mould bar assemblies to ascend the tips and to be tilted backwards. Zinc is dispensed as the mould bars traverse the horizontal region of the runners. The duration of vibration was set by the speed of the conveyor, normally set at 60 bars/minute, and the length of the runners.

The following conditions were used for the Current and Best process selections:

Current Best

Casting Temperature 500°C 500°C

Frequency 50-60 Hz 50-60 Hz

Preheat bar 160°C 280°C

Amplitude 0.2 mm 0.4 mm

Duration 3.5 s 2 s

Calot Weight 5 g 5 g

The results for the Best (LOT 8), Current (LOT 3), and intermediate process values are shown in Table 7:

Table 7

It may be seen that amplitude is the most important factor. In this example, an amplitude of 0.4 mm is superior to 0.2 mm on all metrics. At 0.4 mm amplitude, a 280°C mould bar preheat temperature is superior to 160°C in that it provides a smaller grain size, a lower top edge radius and a better parallel edge length. The 0.4 mm amplitude produces a single example of poor concavity (LOT 4) when combined with a low preheat temperature and a long duration.

Thus, applying the 'Best' process values enables the following preferred attributes to be achieved in a cast calot:

Grain Size <0.2 mm Top Edge Radius <0.5 mm Parallel Edge Length >85% Concavity <0.4 mm

Example 4

Extrusion trials were carried out to compare LOT 8 calots ('Best') with LOT 3 calots ('Current') from Example 3, under identical impact extrusion conditions. The resulting cans were assessed for longitudinal can splits, as shown in Table 8.

Table 8

Table 8 shows that LOT 8 calots reduce can splitting by an order of magnitude compared with LOT 3 calots, due to a more refined grain size and a less segregated lead distribution, and due to having a more regular shape.

Example 5

A vibrator arm assembly was used to transmit a constant amplitude vibration throughout a predetermined duration measured from the point of zinc dispense. The length of time was dictated by the length of a pair of parallel runners in contact with the underside of the carrier bars and by the speed of the casting conveyor. Thus, for a 2 second duration, when the conveyor was set at the desirable casting speed of 60 bars/minute, the length was equivalent to the width of 2 mould bars = 36.5 mm. Vibration was commenced just prior to zinc dispense to ensure that the vibrations were active through the molten zinc when the first zinc dendrites were starting to solidify, and thereby disrupted immediately. The mould bars were maintained horizontal throughout the complete solidification process to maintain parallel faces to the calot. The process conditions for three processes were as follows:

VARIABLE Process A Process B Process C

Vibration amplitude 0.1mm 0.2-0.3mm 0.2-0.3mm

Vibration duration 0.5-1 sec* 1.5-2.0sec* 1.5-2.0sec*

Mould bar temperature 110°C 225°C 110°C

Zinc dispense tank temp 500°C 505°C 505°C

Calot weight 5.0 +/-0.2g 5.0 +/-0.2g 5.0 +/-0.2g

* depending on mould position in front or rear row of cavities within mould bar.

Calot profile and microstructure were assessed. The results are shown in Table 9: Table 9

Table 9 shows that the overall calot shape and microstructure for Process A was poor in comparison to Processes B and C, due to insufficient vibration amplitude and duration to enable grain and shape refinement during the solidification. Process B compared with Process C demonstrates the effect of preheating the mould bar in extending the solidification time window for permitting grain and shape refinement. Process B produced excellent grain refinement, calot shape and lead distribution.

The split rate below the trim line was measured for cans formed from the calots prepared according to each of the three processes, based on 20,000 cans in each case. The results are shown in Table 10 below:

Table 10

Example 6:

Calots for C-size battery cans were cast from molten zinc into preheated moulds vibrated at mains frequency at an amplitude of 0.4 mm, under variable preheat temperatures. In each case, a casting temperature of 500°C was used, and vertical vibrations were applied to the mould bar carrying the calot casts. The effect of varying the delay period from zinc dispense and the duration of the vibration on various calot metrics was determined.

The solidified calots were analysed for the presence of bubbles, surface holes, shape, grain refinement and lead distribution, and ascribed values from 0 to 3 (whereby 0 represents best result, 3 represents worst result, and 1 and 2 represent intermediate results).

The results are shown in Table 11 below:

Table 11

Claims

CLAIMS:

1. A method of producing a cast zinc calot having a fine, equiaxed, noncolumnar grain microstructure, which comprises applying vibrations to molten zinc in a mould during cooling, wherein the application of the vibrations is initiated prior to or shortly after the onset of solidification and continued during solidification such that the growth of dendrites in the zinc is disrupted.

2. A method according to claim 1 wherein the vibrations are sufficient to disrupt surface tension forces at the zinc/air/mould boundaries.

3. A method according to claim 1 or claim 2 wherein the vibrations provide a cast calot having a microstructure of grain size less than 0.2 mm.

4. A method according to any preceding claim wherein the vibrations provide a cast calot having a top edge radius of less than 0.5 mm.

5. A method according to any preceding claim wherein the vibrations are applied in an amplitude greater than 0.2 mm at a frequency in the range 50 to 60 Hz.

6. A method according to any of claims 1 to 5 wherein the application of the vibrations is initiated prior to the onset of solidification.

7. A method according to any of claims 1 to 5 wherein the application of the vibrations is initiated shortly after the onset of solidification.

8. A method according to any preceding claim wherein the vibrations are applied at a frequency of 50 Hz in an amplitude of at least 0.4 mm for a duration of from 0.5 to 2.5 seconds, or in an amplitude of at least 0.3 mm for a duration of from 1.0 to 2.5 seconds.

9. A method according to any preceding claim wherein the mould is preheated to a temperature above 160°C.

10. A method according to claim 9 wherein the mould is preheated to a temperature above 250°C.

11. A method according to any preceding claim, further comprising the step of impact extruding the cast zinc calot to form a zinc battery can.

12. A cast zinc calot having a fine, equiaxed, noncolumnar grain microstructure.

13. A cast zinc calot according to claim 12 comprising an isotropic lead particle distribution that is intrinsically linked to the grain microstructure.

14. A cast zinc calot as defined in claim 12 or claim 13 produced by a method according to any of claims 1 to 10.

15. A zinc battery can formed by the method according to claim 11.